Galilei (February 15, 1564 - January 8, 1642), was an Italian astronomer, philosopher,
and physicist who is closely associated with the Scientific Revolution. He has
been referred to as the "father of modern astronomy" (a title to which Kepler
has perhaps a stronger claim), as the "father of modern physics", and as "father
of science". His experimental work is widely considered complementary to the writings
of Bacon in establishing the modern scientific method. Galileo was born in Pisa
and his career coincided with that of Kepler. The work of Galileo is considered
to be a significant break from that of Aristotle; in particular, Galileo placed
emphasis on quantity, rather than quality.
the pantheon of the scientific revolution Galileo occupies a high position because
of his pioneering use of quantitative experiments with results analyzed mathematically.
There was no tradition of such methods in European thought at that time; the great
experimentalist who immediately preceded Galileo, William Gilbert, did not use
a quantitative approach. (However, Galileo's father, Vincenzo Galilei, had performed
experiments in which he discovered what may be the oldest known non-linear relation
in physics, between the tension and the pitch of a stretched string.)
20th century the reality of Galileo's experiments was challenged by some authorities,
in particular the distinguished French historian of science Alexandre Koyré. The
experiments reported in Two New Sciences to determine the law of acceleration
of falling bodies, for instance, required accurate measurements of time, which
appeared to have been impossible with the technology of 1600. According to Koyré,
the law was arrived at deductively, and the experiments were merely illustrative
Later research, however, has validated the experiments.
The experiments on falling bodies (actually rolling balls) were replicated using
the methods described by Galileo (Settle, 1961), and the precision of the results
was consistent with Galileo's report. Later research into Galileo's unpublished
working papers from as early as 1604 clearly showed the reality of the experiments
and even indicated the particular results that led to the time-squared law (Drake,
Galileo was one of the first people to use the
telescope to observe the sky. Based on sketchy descriptions of existing telescopes,
he made one with about 8x magnification, and then made improved models up to about
20x. He published his initial telescopic observations in March 1610 in a short
treatise entitled Sidereus Nuncius (Sidereal Messenger).
Galileo Galilei’s discovery of the moons of Jupiter. This
is a manuscript page, in Italian, on which Galileo first noted an observation
of the moons; a full description of them appeared in Sidereus Nuncius in March
1610. For a translation from Sidereus Nuncius click on the picture.
1610 Galileo discovered Jupiter's four largest satellites (moons): Io, Europa,
Ganymede, and Callisto. He determined that these moons were orbiting the planet
since they would occasionally disappear; something he attributed to their movement
behind Jupiter. He made additional observations of them in 1620. (Later astronomers
overruled Galileo's naming of these objects, changing his Medicean stars
to Galilean satellites.) The demonstration that a planet had smaller
planets orbiting it was problematic for the orderly, comprehensive picture of
the geocentric model of the universe, in which everything circled around the Earth.
Galileo noted that Venus exhibited a full set of phases like the Moon. Because
the apparent brightness of Venus is nearly constant, Galileo reasoned that Venus
could not be circling the Earth at a constant distance. By contrast, the heliocentric
model of the solar system developed by Copernicus would neatly account for the
steady brightness by reason of the much greater distance from the Earth at the
time of "full Venus", when the two planets were on opposite sides of the sun such
that Venus' illuminated hemisphere faced the Earth.
Galileo made the first
European observations of sunspots, although there is evidence that Chinese astronomers
had done so before him. The very existence of sunspots showed another difficulty
with the perfection of the heavens as assumed in the older philosophy. And the
annual variations in their motions, first noticed by Francesco Sizzi, presented
great difficulties for either the geocentric system or that of Tycho Brahe.
was the first to report lunar mountains, whose existence he deduced from the patterns
of light and shadow on the Moon's surface. He even estimated their heights from
these observations. This led him to the conclusion that the Moon was "rough and
uneven, and just like the surface of the Earth itself", and not a perfect sphere
as Aristotle had claimed.
Galileo observed Neptune in 1611, but believed it
to be a star.
Galileo's theoretical and experimental work
on the motions of bodies, along with the largely independent work of Kepler and
Descartes, was a precursor of the Classical mechanics developed by Sir Isaac Newton.
He was a pioneer, at least in the European tradition, in performing rigorous experiments
and insisting on a mathematical description of the laws of nature.
One of the
most famous stories about Galileo is that he dropped balls of different masses
from the Leaning Tower of Pisa to demonstrate that their velocity of descent was
independent of their mass (excluding the limited effect of air resistance). This
was contrary to what Aristotle had taught: that heavy objects fall faster than
lighter ones, in direct proportion to weight. Though the story of the tower first
appeared in a biography by Galileo's pupil Viviani, it is now not generally believed
to be true. However, Galileo did do experiments involving balls rolling down inclined
planes, which showed the same thing. He determined the correct mathematical law
for acceleration: the total distance covered, starting from rest, is proportional
to the square of the time. He concluded that falling objects are accelerated independently
of their mass, and that objects retain their velocity unless a force acts upon
Galileo also noted that a pendulum's swings always take the same amount
of time, independently of the amplitude. While Galileo believed this equality
of period to be exact, it is only approximate, applying to small swings. It is
good enough to regulate a clock, however, as Galileo may have been the first to
realize. (See Technology.)
In the early 1600s, Galileo and an assistant tried
to measure the speed of light. They stood on different hilltops, each holding
a shuttered lantern. Galileo would open his shutter, and, as soon as his assistant
saw the flash, he would open his shutter. At a distance of less than a mile, Galileo
could detect no delay in the round-trip time greater than when he and the assistant
were only a few yards apart. While he could reach no conclusion on whether light
propagated instantaneously, he recognized that the distance between the hilltops
was perhaps too small for a good measurement.
Galileo's application of mathematics to experimental physics was innovative, his
mathematical methods were the standard ones of the day. The analyses and proofs
relied heavily on the Eudoxian theory of proportion, as set forth in the fifth
book of Euclid's Elements. This theory had become available only a century before,
thanks to accurate translations by Tartaglia and others; but by the end of Galileo's
life it was being superseded by the algebraic methods of Descartes, which a modern
finds incomparably easier to follow.
Galileo produced one piece of original
and even prophetic work in mathematics: Galileo's paradox, which shows that there
are as many perfect squares as there are whole numbers, even though most numbers
are not perfect squares. Such seeming contradictions were brought under control
250 years later in the work of Georg Cantor.
made a few contributions to what we now call technology as distinct from pure
physics, and suggested others. This is not the same distinction as made by Aristotle,
who would have considered all Galileo's physics as techne or useful knowledge,
as opposed to episteme, or philosophical investigation into the causes
In 1595 - 1598 Galileo devised and improved a "Geometric and Military
Compass" suitable for use by gunners and surveyors. This expanded on earlier instruments
designed by Tartaglia and Guidobaldo. For gunners, it offered, in addition to
a new and safer way of elevating cannon accurately, a way of quickly computing
the charge of gunpowder for cannonballs of different sizes and materials. As a
geometric instrument it enabled the construction of any regular polygon, computation
of the area of any polygon or circular sector, and a variety of other calculations.
About 1606 - 1607 (or possibly earlier) Galileo made a thermometer, using the
expansion and contraction of air in a bulb to move water in an attached tube.
In 1610 he used a telescope as a compound microscope, and he made improved
microscopes in 1623 and after. This appears to be the first clearly documented
use of the compound microscope.
In 1612, having determined the orbital periods
of Jupiter's satellites, Galileo proposed that with sufficiently accurate knowledge
of their orbits one could use their positions as a universal clock, and this would
make possible the determination of longitude. He worked on this problem from time
to time during the rest of his life; but the practical problems were insurmountable,
and it was another century before John Harrison mastered longitude with his chronometer.
In his last year, when totally blind, he designed an escapement mechanism for
a pendulum clock. The first fully operational pendulum clock was made by Huygens
in the 1650s.
He created sketches of various inventions, such as a candle and
mirror combination to reflect light throughout a building, an automatic tomato
picker, a pocket comb that doubled as an eating utensil, and what appears to be
a ballpoint pen.
Galileo was a devout Catholic,
yet his writings on Copernican heliocentrism disturbed the Catholic Church, which
believed in a geocentric model of the solar system. The church argued that heliocentrism
was in direct contradiction of the Bible and the highly revered ancient writings
of Aristotle and Plato. For his insights, Galileo was threatened with death at
the stake and would eventually face lifelong house arrest after recanting his
The geocentric model was generally accepted at the time not only for
scriptural reasons. By the time of the controversy, the Catholic Church had in
fact abandoned the Ptolemaic model for the Tychonian model in which the Earth
was at the centre of the Universe, the Sun revolved around the Earth and the other
planets revolved around the Sun. This model is geometrically equivalent to the
Copernican model and had the extra advantage that it predicted no parallax of
the stars, an effect that was impossible to detect with the instruments of the
An understanding of the controversies, if it is even possible, requires
attention not only to the politics of religious organizations but to those of
academic philosophy. Before Galileo had trouble with the Jesuits and before the
Dominican friar Caccini denounced him from the pulpit, his employer heard him
accused of contradicting Scripture by a professor of philosophy, Cosimo Boscaglia,
who was neither a theologian nor a priest. The first to defend Galileo was a Benedictine
abbot, Benedetto Castelli, who was also a professor of mathematics and a former
student of Galileo's. It was this exchange that led Galileo to write the Letter
to Grand Duchess Christina. (Castelli remained Galileo's friend, visiting
him at Arcetri near the end of Galileo's life, after months of effort to get permission
from the Inquisition to do so.)
However, real power lay with the Church, and
Galileo's arguments were most fiercely fought on the religious level. The late
nineteenth and early twentieth century historian Andrew Dickson White wrote from
an anti-clerical perspective:
The war became more and more bitter.
The Dominican Father Caccini preached a sermon from the text, "Ye men of Galilee,
why stand ye gazing up into heaven?" and this wretched pun upon the great astronomer's
name ushered in sharper weapons; for, before Caccini ended, he insisted that "geometry
is of the devil," and that "mathematicians should be banished as the authors of
all heresies." The Church authorities gave Caccini promotion. Father Lorini proved
that Galileo's doctrine was not only heretical but "atheistic," and besought the
Inquisition to intervene. The Bishop of Fiesole screamed in rage against the Copernican
system, publicly insulted Galileo, and denounced him to the Grand-Duke. The Archbishop
of Pisa secretly sought to entrap Galileo and deliver him to the Inquisition at
Rome. The Archbishop of Florence solemnly condemned the new doctrines as unscriptural;
and Paul V, while petting Galileo, and inviting him as the greatest astronomer
of the world to visit Rome, was secretly moving the Archbishop of Pisa to pick
up evidence against the astronomer. But by far the most terrible champion who
now appeared was Cardinal Bellarmin, one of the greatest theologians the world
has known. He was earnest, sincere, and learned, but insisted on making science
conform to Scripture. The weapons which men of Bellarmin's stamp used were purely
theological. They held up before the world the dreadful consequences which must
result to Christian theology were the heavenly bodies proved to revolve about
the Sun and not about the Earth. Their most tremendous dogmatic engine was the
statement that "his pretended discovery vitiates the whole Christian plan of salvation."
Father Lecazre declared "it casts suspicion on the doctrine of the incarnation."
Others declared, "It upsets the whole basis of theology. If the Earth is a planet,
and only one among several planets, it can not be that any such great things have
been done specially for it as the Christian doctrine teaches. If there are other
planets, since God makes nothing in vain, they must be inhabited; but how can
their inhabitants be descended from Adam? How can they trace back their origin
to Noah's ark? How can they have been redeemed by the Saviour?" Nor was this argument
confined to the theologians of the Roman Church; Melanchthon, Protestant as he
was, had already used it in his attacks on Copernicus and his school. (White,
1898; online text)
In 1616, the Inquisition warned Galileo not
to hold or defend the hypothesis asserted in Copernicus's On the Revolutions,
though it has been debated whether he was admonished not to "teach in any way"
the heliocentric theory. When Galileo was tried in 1633, the Inquisition was proceeding
on the premise that he had been ordered not to teach it at all, based on a paper
in the records from 1616; but Galileo produced a letter from Cardinal Bellarmine
that showed only the "hold or defend" order. The latter is in Bellarmine's own
hand and of unquestioned authenticity; the former is an unsigned copy, violating
the Inquisition's own rule that the record of such an admonition had to be signed
by all parties and notarized. Leaving aside technical rules of evidence, what
can one conclude as to the real events? There are two schools of thought. According
to Stillman Drake, the order not to teach was delivered unofficially and improperly;
Bellarmine would not allow a formal record to be made, and assured Galileo in
writing that the only order in effect was not to "defend or hold". According to
Giorgio di Santillana, however, the unsigned minute was simply a fabrication by
Despite his continued insistence that his work in the area
was purely theoretical, despite his strict following of the church protocol for
publication of works (which required prior examination by church censors and subsequent
permission), and despite his close friendship with Maffeo Barberini who later
became Pope Urban VIII and presided throughout the ordeal, Galileo was forced
to recant his views repeatedly, and was put under life-long house arrest from
1633 to 1642.
The Inquisition had rejected earlier pleas by Galileo to postpone
or relocate the trial because of his ill health. At a meeting presided by Pope
Urban VIII, the Inquisition decided to notify Galileo that he either had to come
to Rome or that he would be arrested and brought there in chains. Galileo arrived
in Rome for his trial before the Inquisition on February 13, 1633. After two weeks
in quarantine, Galileo was detained at the comfortable residence of the Tuscan
ambassador, as a favor to the influential Grand Duke Ferdinand II de' Medici.
In April 1633, he was formally interrogated by the Inquisition. He was not imprisoned
in a dungeon cell, but detained in a room in the offices of the Inquisition for
On June 22, 1633, the Roman Inquisition started its trial against
Galileo, who was then 69 years old and pleaded for mercy, pointing to his "regrettable
state of physical unwellness". Threatening him with torture, imprisonment, and
death on the stake, the show trial forced Galileo to "abjure, curse and detest"
his work and to promise to denounce others who held his prior viewpoint. Galileo
did everything the church requested him to do. That the threat of torture and
death Galileo was facing was a real one had been proven by the church in the earlier
trial against Giordano Bruno, who was burned at the stake in 1600 for holding
a naturalistic view of the Universe.
The tale that Galileo, rising from his
knees after recanting, said "Eppur si muove!" (But it does move!) cannot possibly
be true; to say any such thing in the offices of the Inquisition would have been
a ticket to follow Bruno to the stake. But the widespread belief that the whole
incident is an 18th-century invention is also false. A Spanish painting, dated
1643 or possibly 1645, shows Galileo writing the phrase on the wall of a dungeon
cell. Here we have a second version of the story, which also cannot be true, because
Galileo was never imprisoned in a dungeon; but the painting shows that some story
of "Eppur si muove" was circulating in Galileo's time. In the months immediately
after his condemnation, Galileo resided with Archbishop Ascanio Piccolomini of
Siena, a learned man and a sympathetic host; the fact that Piccolomini's brother
was a military attaché in Madrid, where the painting was made some years later,
suggests that the Archbishop may have related a story to his family, and it later
became garbled in oral tradition.
Galileo was sentenced to prison, but because
of his advanced age (and/or Church politics) the sentence was commuted to house
arrest at his villas in Arcetri and Florence. Because of a painful hernia,
he requested permission to consult physicians in Florence, which was denied by
Rome, which warned that further such requests would lead to imprisonment. Under
arrest, he was forced to recite penitentiary psalms regularly, and his social
contacts were at times highly restricted, but he was allowed to continue his less
Publication was another matter. His Dialogue
had been put on the Index Librorum Prohibitorum, the official black list
of banned books, where it stayed until 1822 (Hellman, 1998). Though the sentence
announced against Galileo mentioned no other works, Galileo found out two years
later that publication of anything he might ever write had been quietly banned.
The ban was effective in France, Poland, and German states, but not in the Netherlands.
He went totally blind in 1638 (his petition to the Inquisition to be released
was rejected, but he was allowed to move to his house in Florence where he was
closer to his physicians).
According to Andrew Dickson White and many of his
colleagues, Galileo's experiences demonstrate a classic case of a scholar forced
to recant a scientific insight because it offended powerful, conservative forces
in society: for the church at the time, it was not the scientific method that
should be used to find truth -- especially in certain areas -- but the doctrine
as interpreted and defined by church scholars, and this doctrine was defended
with torture, murder, deprivation of freedom, and censorship.
the viewpoints of White and his colleagues have become less generally accepted
by the academic community, partially because White wrote from a perspective that
Christianity is a destructive force. This attitude can also be seen in the works
of Bertolt Brecht, whose play about Galileo is one of the chief sources for popular
ideas about the scientist. Moreover, deeper examination of the primary sources
for Galileo and his trial shows that claims of torture and deprivation were likely
exaggerated. Dava Sobel's Galileo's Daughter offers a different set of
insights into Galileo and his world, in large part through the private correspondence
of Maria Celeste, the daughter of the title, and her father.
In 1992, 359 years
after the Galileo trial, Pope John Paul II issued an apology, lifting the edict
of Inquisition against Galileo: "Galileo sensed in his scientific research the
presence of the Creator who, stirring in the depths of his spirit, stimulated
him, anticipating and assisting his intuitions." After the release of this report,
the Pope said further that "... Galileo, a sincere believer, showed himself to
be more perceptive in this regard [the relation of scientific and Biblical truths]
than the theologians who opposed him."
Writings by Galileo
- Dialogue Concerning the Two Chief World Systems
- The Starry (Sidereal)
- Letter to Grand Duchess Christina
See also: Galilean transformation, Lorentz transformation
- Drake, Stillman (1973). "Galileo's
Discovery of the Law of Free Fall". Scientific American v. 228, #5, pp.
- Drake, Stillman (1978). Galileo At Work. Chicago: University
of Chicago Press. ISBN 0-226-16226-5
- Hellman, Hal (1988). Great Feuds
in Science. Ten of the Liveliest Disputes Ever. New York: Wiley, 1998.
Thomas B. (1961). "An Experiment in the History of Science". Science,
- White, Andrew Dickson (1898). A History of the Warfare of Science
with Theology in Christendom. New York 1898. Public domain text, full online version.